In known designs of such an after-treatment system, a urea injector is disposed downstream of a particulate filter to spray urea solution into a mixing zone where the urea is intended to completely evaporate and mix throughly with engine exhaust that has been filtered by the particulate filter. Upon leaving the mixing zone, the mixture is directed across surfaces of an SCR catalyst where oxides of nitrogen (NOx) are chemically reduced via catalytic action.
In one specific design of such an after-treatment system, the injector nozzle is aimed at a right angle to the direction of exhaust flow to direct the injected urea solution across the entire exhaust flow entering the mixing zone, with the nozzle being disposed at one wall, specifically a top wall and aimed toward an opposite wall, specifically a bottom wall. The inventor has observed that this specific design has a tendency to form crystalline urea deposits on a bottom wall of the mixing zone and on the surface of a boss where the urea injector is mounted. The accumulation of such deposits is undesirable.